As well known to those skilled in the art, molten metal maintained at a prescribed temperature passes between rotating rolls of a twin roll strip casting apparatus so that a strip having a thickness desired by users is manufactured. In the case of casting a strip having such desired thickness, it is very important to accurately control a gap defined between the rolls of the twin roll strip casting apparatus. Moreover, it is very important to safely draw the cast strip to a coiler.
FIG. 1 is a schematic view illustrating a strip casting process in a general twin roll strip casting apparatus. As shown in FIG. 1, the strip casting process is carried out from a molten metal receiving space 7 defined between two rolls of the twin roll strip casting apparatus, which are rotated in opposite directions, such as a stationary roll 1 and a movable roll 2. When a stopper 5 is moved upward so that the stopper 5 is disengaged from a tundish hole 6 of a tundish 3, the molten metal is supplied from the tundish 3 to the molten metal receiving space 7 defined between the stationary roll 1 and the movable roll 2 through the tundish hole 6 and a nozzle 4. The molten metal supplied to the molten metal receiving space 7 is solidified between the rolls 1 and 2 within 0.2 second. The solidified metal is rolled to form a strip 10, which is wound onto a coiler 12 via a discharge line 11. The height of the molten metal is detected by means of a height-detecting sensor 8 for detecting the height of the molten metal. It is basically considered that the height of the molten metal reaches a target value immediately after the strip casting process is initiated.
According to the conventional method for drawing a strip initially cast by means of the twin roll strip casting apparatus, the strip having passed between the rolls of the twin roll strip casting apparatus is drawn via the discharge line 11. When the strip casting process is initiated, the stopper 5 is moved upward so that the stopper 5 is disengaged from the tundish hole 6 of the tundish 3. Consequently, the molten metal is supplied from the tundish 3 to the molten metal receiving space 7. At this time, a first portion of the molten metal passes between the rolls 1 and 2 while being solidified. Subsequently, the solidified strip 10, which follows the first solidified portion of the molten metal, is successively rolled between the rolls 1 and 2, drawn along the discharge line 11, and wound onto the coiler 12.
In the conventional method for drawing the initially cast strip, however, timing of the drawing is not accurately established. In the case that a solidification transition procedure of the molten metal is unstable as described above, all subsequent normal operations may not be carried out. Especially when excessive solidification of the strip occurs in the course of initial solidification, the rolls may be damaged. When the strip is not appropriately solidified, on the other hand, the strip may be broken, or the molten metal may flow out, which leads to suspension of the operations. Furthermore, a leader strip may be molten due to the molten metal having a high temperature in the case that the above-mentioned procedure is not smoothly carried out.
The process of drawing the initially cast strip, which is carried out when the strip casting process is initiated, is very important, and therefore a method for efficiently carrying out the above-mentioned process is increasingly required.